The invention relates to an electrochemical sensor for the detection of nitrogen oxides.
The German unexamined specification No. 2,304,464 describes a sensor for monitoring the function of catalysts in the exhaust gas cleaning systems of internal combustion engines. The sensor consists of an oxygen or .lambda. probe. If the catalyst is poisoned or if for some other reason, such as aging, the catalyst is not capable of functioning fully to oxidize or reduce, respectively, hydrocarbons, carbon monoxide, or nitrogen oxides, a higher oxygen concentration will appear downstream for the catalyst, since the oxygen will then not be consumed catalytically. In the said known arrangement the sensor is arranged downstream from the catalyst and measures the oxygen coming from the catalyst. On reaching a certain, preset value, a signal is then produced which indicates that the catalyst is no longer, or no longer sufficiently, active.
The ability of a catalyst to convert hydrocarbons, carbon monoxide and oxides of nitrogen into less harmful gases by oxidation is not ascertained directly in the known device, but rather indirectly by measuring the oxygen still appearing downstream from the catalyst. This method is very inaccurate and is unsuitable for practical application, since the oxygen concentration downstream from the catalyst is not only dependent on the function of the catalyst by itself but furthermore on the manner of operation of the engine in the various load ranges. Moreover the measurement of the oxygen concentration downstream from the catalyst may not be used for evaluating the function of the catalyst, because such data do not represent the nitrogen oxide concentration downstream from the catalyst. Accordingly, the known device is not able to reliably monitor the function of a catalyst.
The German unexamined specification No. 2,335,402 describes a sensor for the electrochemical determination of the content of nitrogen oxides in exhaust gases, which amongst other things is used to monitor the function of an exhaust gas catalyst. Such a sensor is arranged in an exhaust pipe shunt and the maximum permitted temperature of the known sensor is 70.degree. C. owing to its design and for instance owing to the use of Teflon as a diffusion barrier.
The disadvantage of this device is more especially that the nitrogen oxide fraction in the exhaust gas is not measured downstream from the catalyst but in the shunt or bypass. Downstream from the catalyst there is a temperature of at least 400 degrees in the exhaust system, i. e. in the exhaust gas. The known sensor is in no way able to withstand this temperature. Accordingly the sensor in the shunt in addition has to be cooled, something creating added complexity. It is furthermore possible for the shunt to be shut off intentionally or unintentionally and as a result no reliable monitoring of the catalyst will be possible.
Although there are methods of physical analysis by which nitrogen oxides, hydrocarbons and/or carbon monoxide may be measured at temperatures of 400.degree. C., flame ionisation detectors, infrared analyzers and/or chemoluminescence measuring instruments are used for this purpose, which without exception represent complicated items of apparatus and are in no way suitable for use in exhaust systems both on account of the expense and also as regards the amount of space required, mechanical stability and other factors.